The present invention concerns a process and an installation for removing solvent vapors from exhaust or waste air.
There are various situations in which there is a need for solvent vapor to be removed from exhaust or waste air, for example inter alia in printing works where, in particular in the area of rotary intaglio printing, toluene which is used as a solvent for the printing inks and which in operation of the printing machine is discharged into the air in the working space not only has to be removed from the working space but also separated out of the exhaust air which is sucked out of the working space, and recycled so that the toluene can be used again. A process and installation for performing those operations are to be found for example in `Haus der Technik` lecture publications 404 (1978), pages 98 ff.
The above-described operating procedure is effected by means of activated carbon adsorbers from which the cleaned exhaust air issues in a pratically solvent-free condition. In actual fact the residual solvent content in the exhaust air, at about 20 mg/m.sup.3, is so far below the limit value of 100 mg/m.sup.3 which is required by the present statutory provisions in the Federal Republic of Germany (such air being air to be discharged into the atmosphere by way of a chimney or the like.
As an activated carbon adsorber can remove impurities from the exhaust air which flows therethrough and which requires cleaning therein, only until loading of such impurities has reached a certain limit value, the adsorber must switch between two operating modes. The one operating mode is the above-described charging mode during which the contaminated exhaust air flows through the adsorber in an upward direction, the solvent vapors being adsorptively bound by the activated carbon layers. When the adsorption capacity of the activated carbon is fully used up, the adsorber is automatically separated from the flow of exhaust air and switched over into a regeneration mode in which saturated water vapor or steam is fed to the adsorber by way of a valve so that the vapor flows through the activated carbon in a downward direction, during which time the activated carbon bed is heated and the adsorptively bound solvent is desorbed. The mixture of vapors which is produced in that case is condensed and fed to a piece of equipment in which the solvent is separated from the water and thus maintained in such a pure condition that it can be re-used.
When the adsorber has been regenerated, the feed of water vapor is terminated and the adsorber is put back into the charging mode condition again in which the exhaust air to be cleaned flows through it.
When this operating procedure is used in the context of printing, in order not to disturb continuous operation of the printing machines by virtue of the fact that solvent adsorption is not possible during the regeneration period, the installation involved in carrying out the operating procedure usually has a plurality of adsorbers which are operated in parallel and the number of which is so great that, when one of the adsorbers is operating in the regeneration mode, the adsorption capacity or cleaning capacity of the other adsorbers is sufficient to produce the required cleaning effect for the exhaust air which is to be sucked out of the working space in which the printing equipment is disposed, for removal of the solvent vapors produced by the machines. In that situation the adsorbers are operated in opposite relationship at least from time to time, in such a way that at least one of the adsorbers is in the regeneration mode while at least one other adsorber is operating in the charging mode.
In order to keep the concentration of solvent at the working location below the prescribed values, a considerable amount of air must be removed from a printing room in which one or even a plurality of rotary intaglio printing machines are operating, and that amount of air, per printing machine, may be of the order of magnitude of up to 150,000 m.sup.3 /hour. Admittedly, the air is not sucked away directly in the printing room itself, but rather the air is sucked out of the machine hoods which cover over the rotary intaglio printing machines, or from the drying devices of the printing mechanisms, which however are open on their feed flow side to the printing room. So that the slight reduced pressure which is desired in relation to the outside air is maintained within the printing room, the printing room must be supplied with an amount of conditioned fresh air which corresponds to the amount of air that is sucked out of the printing room. The fresh air is air which is sucked in out of the surrounding atmosphere. That air must be filtered to separate off dust and other solid particles, heated or cooled to the required temperature, and possibly also adjusted to a desired value in respect of its moisture content or humidity. So that those conditioning operations can be carried out for the large amounts of air indicated above, the equipment involved requires correspondingly large fresh air blowers and conditioning installations, and operation thereof involves a considerable level of energy expenditure.